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Security researchers believe they have identified the botnet responsible for a recent spike in traffic on the anonymizing Tor network, but the exact purpose of the malware remains unclear.

On Friday, security firm Fox-IT called out the culprit as a variant of a botnet sometimes known as "Mevade.A", which has been making the rounds under various names since at least 2009.

According to security analysts at Trend Micro, PCs in the US and Japan are most often infected by Mevade, although it is believed to have originated from a Russian-speaking region. It has occasionally been distributed via a malicious program disguised as the Adobe Flash installer.

Once it takes hold on a PC, Mevade downloads various modules, such as adware and browser toolbars, depending on what the botnet's operators want to achieve. According to Fox-IT, infected systems have recently begun downloading components containing Tor functionality, which Mevade then uses to route command and control information through the anonymizing network (rather than using HTTP and other methods, as it had previously).

"The botnet appears to be massive in size as well as very widespread," Fox-IT researchers observed. "Even prior to the switch to Tor, it consisted of tens of thousands of confirmed infections within a limited amount of networks. When these numbers are extrapolated on a per country and global scale, these are definitely in the same ballpark as the Tor user increase."

Fox-IT says it has also confirmed that the version of Tor currently being used by Mevade is version 0.2.3.25. That jibes with findings by the Tor Project's Roger Dingledine, who observed last week that the influx of new Tor clients weren't using the improved Tor handshaking protocol that debuted with version 0.2.4.

Because of the malware's evasive nature, however, just what the Mevade botnet's operators hope to achieve is not altogether clear, although direct or indirect financial gain is a likely goal.

"It is possible that the purpose of this malware network is to load additional malware onto the system and that the infected systems are for sale. We have however no compelling evidence that this is true, so this assumption is merely based on a combination of small hints," Fox-IT researchers wrote.

Trend Micro, however, went as far as to name specific actors as being behind the botnet, saying they are "part of a well organized and well financed cybercrime gang" operating out of Israel and the Ukraine.

"We strongly associate these actors with installations of adware and hijacking search results," Trend Micro senior threat researcher Feike Hacquebord wrote. "Therefore, we suspect that one of the ways the Mevade botnet is monetized is by installing adware and toolbars onto affected systems."

Hacquebord added that unwanted ads aren't Mevade's only danger, either. The software contains a backdoor that can allow attackers to access infected computers, and it can communicate with remote hosts over encrypted links, which creates all sorts of opportunities for data theft.

Fortunately, because Mevade is not a new malware threat, it can be detected and dealt with by most up-to-date antivirus software. How long it will take to clear its traffic off of the Tor network, however, remains to be seen. ®